Early Triassic Sedimentary Environmental Evolution And Its Biological Responses In The Xingyi Region,Guizhou

The Xingyi region along the northwestern margin of the Nanpanjiang Basin in Guizhou,southwestern China is believed to be an ideal site for the research of the Early Triassic sedimentary environmental evolution and its biological responses because of the well-preserved Early Triassic shallow-marine facies records and the processes from biological extinction to revival.The Lower Triassic strata herein consist dominantly of the Feixianguan and Jialingjiang Formations.The former is mainly made up of the restricted platform-tidal flat siliciclastic deposits intercalated with the carbonate deposits;the latter is dominated by the shallow-marine platform and evaporite platform carbonate rocks intercalated with the siliciclastic deposits.The sedimentary textures and structures contain wavy bedding,lenticular bedding,vein bedding,parallel bedding and locally small-scale cross-bedding.The gypsum pseudomorphs,framboidal pyrite and authigenic quartz grains are also identified by using thin-section examination.Different kinds of anachronistic facies indicates different environmental significance.The space distribution characteristics of anachronistic facies show the environmental changes from the improvements to the waning of the marine environments.Anachronistic facies in study area include stromatolites,vermicular limestones,zebra limestones and giant oolitic limestones.The existence of a large number of microbial fossils and framboidal pyrite reflect the anaerobic or aerobic conditions of sea water for the formation of the stromatolites.The rhythmic of zebra limestones may be related to cyclical fluctuations of sea.The existence of a large number of microbial fossils reflect the anaerobic conditions of sea water for the formation of the limestone belt.The cause of the vermicular limestone is more complicated.Layered and irregular granular,elliptical vermicular limestone reflect the anaerobic sedimentary environment.On the contrary,the deformation of cylindrical vermicular limestone reflect aerobic marine environment.Early Triassic marine environment in the study area have more time to improve-deterioration of the evolutionary process.Carbon isotope results revealed three negative drifts and three positive drifts,respectively in the study area during the Early Triassic,and the environmental changes from the improvements to the waning of the marine environments at that time.The sharp changes of the sulfur isotope values(e.g.-25.5‰ to 26.3‰ for the Feixianguan Formation and-3.9‰ to 37.3‰ for the Jialingjiang Formation)suggested the variable marine ecoenvironments during the Early Triassic.There was a trend of rapid increase of 87Sr/86 Sr ratios from 0.707203 to 0.708107,a maximum for the study area from the Phanerozoic onwards.The MgO/CaO ratios show that the climates were once humid during the Griesbachian to the Spathian,i.e.the periods for the deposition of the first member of the Feixianguan Formation and third member of the Jialingjiang Formation,respectively.Till the late Spathian,i.e.the late stage for the deposition of the third member of the Jialingjiang Formation,the climates were relatively dry in the study area.The palaeosalinity results based on MgO/Al2O3 ratios have disclosed that the study area was once in the normal shallow marine environments and not affected by the continental fresh water during the Early Triassic.The carbon isotope of early Triassic not only have drastic fluctuations on the lateral,also show discrete differentiation in the longitudinal.30 of the carbon isotope profiles show significant discrete coefficient(standard deviation)in early Triassic.Marine primary productivity and layered ocean together contributed to early Triassic carbon isotopes on the longitudinal significant changes,however layered ocean is the main reason.Based on the correlation of carbon isotope in vertical and horizontal,the early Triassic marine environment is divided into 10 stages.Three upward migration events of the oxidation-reduction interfaces in the study area during the Early Triassic are dated at N1,i.e.the Permian-Triassic boundary,middle Smithian and middle-late Spathian,respectively.The first two are significant while the last one is faint.The carbon isotope gradient(??13C)values for the euphotic zone are greater than those for the deep-water zone at N2,P2 and P3,implying that the marine primary productivity is the key factor for the carbon isotope gradient values(??13C).It follows that the early Triassic environmental changes display the coincidence with the biological recovery-survival-extinction processes.Several layers of thick 10-30 cm bivalia fossils bed,purple silty mudstone,indicating the marine environment is a brief recovery in early Griesbachian;Purple color stripe and larger bivalia fossils in pewter terrigenous clastic rocks is increased,which instructs the improvement of the shallow marine environment in middle Griesbachian;Pewter silty fine sand-grit and less fossil content,indicating deterioration of the Marine environment from late Griesbachian to early Dienerian;The appearance of biological diversity,framboidal pyrite particle size slightly increased,positive drift of carbon isotope and decrease of sulfur isotope values suggested the environmental get better in mid-late Dienerian;The decrease of biological diversity of bivalia fossils and conodont,negative drift of carbon isotope and increase of sulfur isotope values suggested deterioration of the marine environment in early Smithian;A large number of pyrite in grain limestone,the decrease of biological diversity,negative drift of carbon isotope and increase of sulfur isotope values suggested deterioration of the marine environment in middle Smithian;The existence of deformation of cylindrical vermicular limestone,bivalia fossils,red clasolite layer suggested better marine environment in late Smithian;Microcrystalline limestone and stromatolite,negative drift of carbon isotope and increase of sulfur isotope values suggested deterioration of the marine environment in in early Spathian.Yellow-green mudstone which is in laminated and thin bedded dolomite,the lack of fossils suggested deterioration of the marine environment in late Spathian.